Amperometric gas sensor

ABSTRACT

An amperometric gas sensor for measuring the concentration of an analyte includes: a solid configured as an insulator without being contacted by the analyte and configured for diffusion of the analyte therethrough, the solid including a non-conductive polymer, the solid further configured to increase in electrical conductivity when in contact with the analyte; a working electrode positioned on and in contact with the solid; and a reference electrode positioned on and in contact with the solid, the reference electrode spaced apart and insulated from the working electrode without the solid being contacted by the analyte, the working electrode and the reference electrode configured to measure electrical conductivity of the solid when the solid is in contact with the analyte.

This application is a continuation of and claims priority under 35U.S.C. §120 to copending, commonly assigned U.S. application Ser. No.13/776,953, filed Feb. 26, 2013, which claims priority to U.S.Provisional Application Ser. No. 61/663,754, filed Jun. 25, 2012. Theseapplications are incorporated herein by reference.

TECHNICAL FIELD

This invention relates to amperometric gas sensors.

BACKGROUND

Sterilizations employing vaporized hydrogen peroxide (hereinaftersometimes referred to as VHP) in a vacuum chamber have been used invalidated pharmaceutical aseptic productions and research applications.

SUMMARY

The currently available methods for sensing VHP concentration as asterilization parameter in a vacuum are limited. Electrochemical methodsexist, such as a DragerSensor H₂O₂ LC sensor, manufactured by DärgerwerkAG. & Co. KGaA, Burkard Dillig, 23558 Lübeck, Germany, but this methodemploys liquid-based electrolytes which precludes their use in asterilization conducted in a vacuum. Generally, where solid polymerelectrolytes are employed in electrochemical methods, the ionicconductivity of the polymer is dependent upon the hydration of thepolymer, as with Nafion (an ionomer in the form of a sulfonatedtetrafluorothylene based fluoropolymer-copolymer), or upon the additionof salt moieties, as in high-ionic conductivity polymer electrolytesemployed with solid-state batteries. Neither of these is suitable foruse in a sterilization process conducted in a vacuum. The water employedwith Nafion would evaporate. Failure to attain the apparently mutuallyexclusive properties of high-ionic conductivity and good mechanicalstrength via the formation of salt containing polymers precludes use ofsuch salt containing polymers in sterilizations conducted in a vacuum.Good mechanical strength is needed for electrolytes that will berequired to withstand the vacuum cycling (e.g., lowest pressure may beabout 4 Torr) and elevated temperatures (e.g., about 60° C.) that wouldtypically be used in a sterilization process conducted in a vacuum.These problems are overcome with the present invention.

This invention relates to an amperometric gas sensor for measuring theconcentration of an analyte, comprising: a solid support; and a workingelectrode in contact with the solid support; wherein the analytecomprises a dopant which when in contact with the solid supportincreases the electrical conductivity of the solid support. In anembodiment, the sensor further comprises a reference electrode. In anembodiment, the solid support comprises a polymer. In an embodiment, atleast a portion of the solid support is amorphous. In an embodiment, atleast a portion of the solid support is crystalline. The solid supportmay comprise an insulator or a semi-conductor prior to being contactedby the analyte. While not wishing to be bound by theory, it is believedthat the analyte functions as both the analyte and as a dopant. Thus, byexposing the solid support to the analyte, the solid support isconverted to an electrolyte with sufficient ionic conductivity to allowthe amperometric gas sensor to function as an electrochemical cell. Thiswas unexpected.

In an embodiment, the sensor is relatively small enough so thatuncompensated resistance may be negligible and a two electrode sensorcan be employed. This allows for a simplified sensor design andminimization of stray current pickup.

In any of the above-indicated embodiments, prior to being contacted bythe analyte, the solid support is characterized by the absence of adopant. The term “absence of dopant” refers to a solid support thatcontains no dopant, or contains one or more dopants at a concentrationlevel that would be considered to be only a trace amount or at animpurity level, for example, no more than about 0.1% by weight, or nomore than about 0.01% by weight, or no more than about 0.001% by weight.

In any of the above-indicated embodiments, the solid support ischaracterized by the absence of salt. The term “absence of salt” refersto a solid support that contains no salt, or contains one or more saltsat a concentration level that would be considered to be only a traceamount or at an impurity level, for example, no more than about 0.1% byweight, or no more than about 0.01% by weight, or no more than about0.001% by weight.

In any of the above-indicated embodiments, the solid support comprises aporous solid, the volume of voids in the porous solid divided by thetotal volume of the porous solid being in the range up to about 0.7, orfrom about 0.1 to about 0.7, or from about 0.3 to about 0.65.

In any of the above-indicated embodiments, the solid support comprises apolymer that is a non-conductive polymer prior to being contacted by theanalyte.

In any of the above-indicated embodiments, the solid support comprisespoly (ethylene terephthalate), poly (ethylene oxide),polyvinylidenefluoride, polyethylene, polypropylene,polyethylene-napthlate, polyphenylenesulfide, polycarbonate,polytetrafluoroethylene, polypropylene oxide, acrylic resin,polystyrene, poly(styrene-acrylonitrile),poly(acrylnitrile-butadiene-styrene), polyvinyl chloride, chlorinatedpolyether, poly(chlorotrifluoro ethylene), glass, ceramic, carbon,graphite, or a mixture of two or more thereof.

In any of the above-indicated embodiments, the working electrodecomprises a noble metal, for example, gold, platinum, iridium,palladium, osmium, silver, rhodium, ruthenium, titanium, or a mixture oftwo or more thereof.

In any of the above-indicated embodiments, the reference electrodecomprises a noble metal, for example, gold, platinum, iridium,palladium, osmium, silver, rhodium, ruthenium, titanium, or a mixture oftwo or more thereof.

In any of the above-indicated embodiments, the analyte comprises anoxidizing gas or a reducing gas. The analyte may comprise vaporoushydrogen peroxide, ethylene oxide, ozone, or a mixture of two or morethereof. The analyte may comprise a hydrogen-containing gas. The analytemay comprise atomic hydrogen, hydrogen sulfide, hydrogen sulfite,ammonia, carbon monoxide, oxalic acid, formic acid, ascorbic acid,phosphorous acid, or a mixture of two or more thereof.

In any of the above-indicated embodiments, the solid support comprisespoly(ethylene terphthalate), the working electrode comprises palladium,and the analyte comprises vaporous hydrogen peroxide.

In any of the above-indicated embodiments, the solid support is in theform of a poly (ethylene terephthalate) film with a thickness in therange from about 0.05 to about 0.6 mm, or from about 0.07 to about 0.5mm, or from about 0.1 to about 0.3 mm, or about 0.25 mm.

In any of the above-indicated embodiments, the sensor further comprisesa reference electrode, and the working electrode and the referenceelectrode are formed by sputtering palladium on the solid support, thesolid support comprising a poly(ethylene terephthalate) film.

In any of the above-indicated embodiments, the sensor further comprisesa reference electrode, and the working electrode and the referenceelectrode comprise palladium, the thickness of the reference electrodeand the working electrode being in the range from about 40 to about 150nanometers, or from about 80 to about 120 nanometers, or about 100nanometers.

In any of the above-indicated embodiments, the sensor further comprisesa reference electrode and the solid support comprises a poly (ethyleneterephathalate) film, the working electrode and the reference electrodecomprising palladium electrodes sputtered on the poly (ethyleneterephthalate) film; the thickness of the electrodes being in the rangefrom about 40 to about 150 nanometers, or from about 80 to about 120nanometers, or about 100 nanometers; the thickness of the poly (ethyleneterephthalate) film being in the range from about 0.05 to about 0.6 mm,or from about 0.07 to about 0.5 mm, or from about 0.1 to about 0.3 mm,or about 0.25 mm; and the separation between the electrodes being fromabout 0.7 to about 0.9 mm, or about 0.88 mm.

In any of the above-indicated embodiments, the sensor comprises aworking electrode and a reference electrode, the working electrode beingconnected to a potential control to maintain a stable voltage potentialat the working electrode with respect to the reference electrode. Thereference electrode may be connected to a current amplifier. The currentamplifier may be connected to a current follower to convert gasconcentration-related current to a voltage.

The invention relates to a method for determining the concentration of agaseous analyte in an enclosed space, the method comprising: placing theamperometric gas sensor of any of the above-indicated embodiments in theenclosed space; flowing the analyte in contact with the sensor; anddetermining the concentration of the analyte in the enclosed space usingthe amperometric gas sensor. The analyte may comprise an oxidizing gasor a reducing gas. The analyte may comprise vaporous hydrogen peroxide,ethylene oxide, ozone, or a mixture of two or more thereof. The analytemay comprise a hydrogen-containing gas. The analyte may comprise atomichydrogen, hydrogen sulfide, hydrogen sulfite, ammonia, carbon monoxide,oxalic acid, formic acid, ascorbic acid, phosphorous acid, or a mixtureof two or more thereof. The pressure within the enclosed space may bebelow atmospheric pressure (e.g., about 0.1 to about 750 Torr),atmospheric pressure, or above atmospheric pressure (e.g., absolutepressure of about 1 to about 2 atmospheres).

The invention relates to a method for determining the concentration of asterilant gas in a vacuum chamber during a sterilization process, themethod comprising: placing the amperometric gas sensor of any of theabove-indicated embodiments in the vacuum chamber; conducting thesterilization process in the vacuum chamber under a vacuum using thesterilant gas; and determining the concentration of the sterilant gas inthe vacuum chamber using the amperometric gas sensor. In an embodiment,the sterilant gas flows in the vacuum chamber and contacts theamperometric gas sensor. In an embodiment, the sterilant gas is mixedwith a carrier gas to form a gaseous mixture, and the gaseous mixtureflows in the vacuum chamber and contacts the amperometric sensor. In anembodiment, pulses of the sterilant gas flow into the vacuum chamber andcontact the amperometric gas sensor. In an embodiment, pulses of agaseous mixture comprising the sterilant gas and a carrier gas flow intothe vacuum chamber and contact the amperometric gas sensor. In any ofthe above-indicated embodiments, the sterilant gas may comprise vaporoushydrogen peroxide, ethylene oxide, ozone, or a mixture of two or morethereof. The sterilant gas may further comprise an alkaline gas. Thesterilant gas may comprise a mixture of vaporous hydrogen peroxide andammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings all parts and features have like references. Anumber of the annexed drawings are schematic illustrations which are notnecessarily proportioned accurately or drawn to scale.

FIG. 1 is a perspective view of an amperometric gas sensor according toan embodiment of the invention.

FIG. 2 is a top plan view of an amperometric gas sensor accordinganother embodiment of the invention.

FIG. 3 is a block diagram of a control circuit for use with anamperometric gas sensor according to the invention.

FIG. 4 is a chart showing the performance of the amperometric gas sensordisclosed in the Example with current levels determined using theamperometric gas sensor compared to hydrogen peroxide concentrationlevels determined using an IR spectrophotometer.

FIG. 5 is a chart showing the performance of the amperometric gas sensordisclosed in the Example wherein current levels determined by theamperometric sensor are compared to pressure levels within the vacuumchamber of the sterilizer used in the Example.

FIGS. 6-9 are charts showing the performance of the amperometric gassensor disclosed in the Example with current levels determined with theamperometric gas sensor being compared to hydrogen peroxideconcentration levels within the vacuum chamber determined using an IRspectrophotometer.

FIGS. 10-12 are charts disclosing equations provided in the Example forconcentration levels of hydrogen peroxide as determined by an IRspectrophotometer and as a function of Δ current determined by theamperometric gas sensor, as well as the pressure Δ as a function of theΔ current. The disclosed values are calculated with Excel and Matlab forthe period under vacuum. Excel is a software program available fromMicrosoft. Matlab is a software program available from MathWorks ofNatich, Mass. The charts in FIGS. 10-12 use the raw data in FIGS. 6-8,respectively

FIGS. 13-15 are charts disclosing equations determined in the Examplefor the Δ in current in pulse 4 as a function of the grams of hydrogenperoxide injected into the sterilizer. The values plotted in FIGS. 13-15are taken from the raw data disclosed in FIGS. 6-8, respectively. Theexponential function is fitted using Excel.

FIG. 16 is a linear-log graph representation which shows a straight lineresponse for the results from FIGS. 10-15.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

All ranges and ratio limits disclosed in the specification and claimsmay be combined in any manner. It is to be understood that unlessspecifically stated otherwise, references to “a,” “an,” and/or “the” mayinclude one or more than one, and that reference to an item in thesingular may also include the item in the plural.

The phrase “and/or” should be understood to mean “either or both” of theelements so conjoined, i.e., elements that are conjunctively present insome cases and disjunctively present in other cases. Other elements mayoptionally be present other than the elements specifically identified bythe “and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

The word “or” should be understood to have the same meaning as “and/or”as defined above. For example, when separating items in a list, “or” or“and/or” shall be interpreted as being inclusive, i.e., the inclusion ofat least one, but also including more than one, of a number or list ofelements, and, optionally, additional unlisted items. Only terms clearlyindicated to the contrary, such as “only one of” or “exactly one of,” ormay refer to the inclusion of exactly one element of a number or list ofelements. In general, the term “or” as used herein shall only beinterpreted as indicating exclusive alternatives (i.e. “one or the otherbut not both”) when preceded by terms of exclusivity, such as “either,”“one of,” “only one of,” or “exactly one of.”

The phrase “at least one,” in reference to a list of one or moreelements, should be understood to mean at least one element selectedfrom any one or more of the elements in the list of elements, but notnecessarily including at least one of each and every elementspecifically listed within the list of elements and not excluding anycombinations of elements in the list of elements. This definition alsoallows that elements may optionally be present other than the elementsspecifically identified within the list of elements to which the phrase“at least one” refers, whether related or unrelated to those elementsspecifically identified. Thus, as a non-limiting example, “at least oneof A and B” (or, equivalently, “at least one of A or B,” or,equivalently “at least one of A and/or B”) can refer, in one embodiment,to at least one, optionally including more than one, A, with no Bpresent (and optionally including elements other than B); in anotherembodiment, to at least one, optionally including more than one, B, withno A present (and optionally including elements other than A); in yetanother embodiment, to at least one, optionally including more than one,A, and at least one, optionally including more than one, B (andoptionally including other elements); etc.

The transitional words or phrases, such as “comprising,” “including,”“carrying,” “having,” “containing,” “involving,” “holding,” and thelike, are to be understood to be open-ended, i.e., to mean including butnot limited to.

The term “sterilization” refers to rendering a substance incapable ofreproduction, metabolism and/or growth. The term “sterilization”includes microbial deactivation. While sterilization is often taken torefer to a total absence of living organisms, the term may be usedherein to refer to a substance free from living organisms to a degreeagreed to be acceptable. The term “sterilization” may be used herein toalso refer to processes less rigorous than sterilization, for example,disinfection, sanitization, decontamination, cleaning, and the like.Variations of the term “sterilization,” such as sterilant, sterilizing,etc., may also be used herein to refer to and encompass related variantsassociated with sterilization processes as well as processes lessrigorous than sterilization (e.g., disinfectant, disinfecting, etc.).

The term “non-conductive polymer” is used herein to refer to a polymeror copolymer characterized by a volume resitivity of at least about 10⁵ohm-cm, or at least about 10⁶ ohm-cm, or at least about 10⁷ ohm-cm, orat least about 10⁸ ohm-cm, or at least about 10⁹ ohm-cm, or at leastabout 10¹⁰ ohm-cm, or at least about 10¹¹ ohm-cm, or at least about 10¹²ohm-cm, or at least about 10¹³ ohm-cm, as determined by ASTM D257-07.Specific non-conductive polymers and copolymers that may be used, andtheir corresponding volume resistivity values include:

Polymer Volume Resistivity (ohm-cm) Polyethylene Low density 10¹⁵-10¹⁸Polyethylene Medium density 10¹⁵-10¹⁸ Polyethylene High density     6 ×10¹⁵-10¹⁸ Polypropylene 6.5 × 10¹⁶ Acrylic resins >10¹⁴ High impactAcrylic resins 10¹⁶-10¹⁷ Polystyrene 10¹⁷-10²¹ Polystyrene high Impactresin 10¹³-10¹⁷ Poly(styrene-acrylonitrile)   10¹⁵Poly(acrylonitrile-butadiene-styrene) 10¹²-10¹⁷ Polyvinyl chloride >10⁹ Chlorinated polyether 1.5 × 10¹⁶ Poly(chlorotrifluoroethylene)   10¹⁸Fluorinated poly(ethylene-propylene) >10¹⁸ Poly(ethylene terephthalate)>10¹⁴ Polycarbonate 1.7 × 10⁵ 

The term “solid support” refers to a solid material that is used tosupport an electrode. The solid support may comprise a solid electrolytewhen contacted by an analyte that comprises a dopant. The solid supportmay be an insulator or a semi-conductor and be transformed to a solidelectrolyte upon being contacted with an analyte that comprises adopant.

The term “solid electrolyte” refers to a solid material which conductselectricity when placed between electrodes and a voltage is applied.

The term “dopant” refers to a material which when in contact with asolid support increases the electrical conductivity of that solidsupport. A dopant may contact a solid support and oxidize (p-doping) orreduce (n-doping) the solid support.

The term “amorphous solid” is used herein to refer to a solid materialthat lacks the long-range order characteristic of a crystal. Amorphoussolids may include ceramics, glass, polymers, nanostructured materials,and the like.

The term “insulator” is used herein to refer to a material whoseinternal electric charges do not flow freely and therefore do notconduct an electric current under the influence of an electric field.There are no perfect insulators, but glass, paper and some polymerswhich have high resistivity characteristics are considered to beinsulators for purposes of this invention. The non-conductive polymersreferred to above are insulators for purposes of this invention.

The term “semiconductor” is used herein to refer to a material which hasan electrical conductivity between that of a conductor such as copperand an insulator such as glass. Current conduction in a semiconductormay occur via free electrons and “holes,” collectively known as chargecarriers. The doping of a semiconducting material may increase thenumber of charge carriers within it. When a doped semiconductor containsexcess holes it may be called “p-type,” and when it contains excess freeelectrons it may be called “n-type.” A single semiconductor crystal canhave multiple p- and n-type regions.

The term “electricity” is used herein to refer to the motion of ionsand/or electrons in an electric field.

The term “electrical conductivity” refers to ionic conductivity and/orelectronic conductivity.

The term “vacuum” is used herein to refer to a pressure that is belowatmospheric pressure. The pressure, in terms of absolute pressure, inthe vacuum may be in the range from about 0.1 to about 750 Torr, or fromabout 0.1 to about 700 Torr, or from about 0.1 to about 600 Torr, orfrom about 0.1 to about 500 Torr, or from about 0.1 to about 400 Torr,or from about 0.1 to about 300 Torr, or from about 0.1 to about 200Torr, or from about 0.1 to about 100 Torr, or from about 1 to about 75Torr, or from about 1 to about 50 Torr, or from about 1 to about 25Torr, or from about 3 to about 25 Torr, or from about 5 to about 25Torr, or from about 5 Torr to about 20 Torr.

Referring to the drawings, FIG. 1 is a perspective view of anamperometric gas sensor in accordance with the present invention.Amperometric gas sensor 10 includes solid support 22, and electrodes 24Aand 24B positioned on the solid support 22. The electrode 24A is aworking electrode. The electrode 24B is a reference electrode. A spaceor gap 26 separates the electrodes 24A and 24B.

An alternate embodiment of the inventive amperometric gas sensor isillustrated in FIG. 2. Referring to FIG. 2, amperometric gas sensor 40includes solid support 42 and electrodes 44 and 46 positioned on thesolid support 42. Electrode 44 is a working electrode which includeselongated straight strip portion 44A and an L-shaped arm portion 44Bthat extends from strip portion 44A. Electrode 46 is a referenceelectrode which includes elongated straight strip portion 46A and anL-shaped arm portion 46B that extends from strip portion 46A. Electrodes44 and 46 are positioned on solid support 42 such that arm portion 44Bof electrode 44 is between strip portion 46A and arm portion 46B ofelectrode 46, and arm portion 46B of electrode 46 is between stripportion 44A and arm portion 44B of electrode 44. A space or gap 48separates electrodes 44 and 46. The electrodes 44 and 46 may beinterdigitally spaced in order to achieve higher signal strength in asmall sensor area.

Referring to FIG. 3, a block diagram of a control circuit 60 forcontrolling amperometric gas sensor 10 is shown. Sensor 10 isillustrated in FIG. 3, but amperometric gas sensor 40 could besubstituted for sensor 10. Control circuit 60, known as a potentiostat,is shown for a two electrode sensor configuration. Control circuit 60includes potential control 62, current follower 72, and currentamplifier 82. Potential control 62 may be provided to maintain a stablevoltage potential at the working electrode with respect to the referenceelectrode at the formal reduction potential necessary for the desiredreaction to occur. The reduction voltage may be minimal, i.e., about0.682 V for hydrogen peroxide. Current follower 72 may be provided toconvert the gas concentration-related current from sensor 10 to avoltage and to process further signal processing. Current amplifier 82may be provided to enable measuring of low-level currents of the nA andpA ranges. DC power for sensor control circuit 60 may be a battery or anAC adapter.

In an embodiment, the electrodes shown in FIGS. 1 and 2 may be formedusing laser ablation techniques. The use of laser ablation allows forthe creation of extremely small feature sizes to be accuratelymanufactured in a repeated manner. The sensors disclosed in FIGS. 1 and2 may be formed by scribing sputtered films (e.g., palladium sputteredpoly(ethylene terphthalate) films) using laser ablation.

In an embodiment, the electrodes of the inventive amperometric sensormay be supported and surrounded by the solid support, except for theends or edges of the electrodes which may remain free for exposure tothe analyte and for connection to the potentiostat circuit. Theelectrodes may be wires that are relatively small in diameter andlength. They may be placed parallel to each other and as close aspossible so that uncompensated resistance along the current path isinsignificant. The solid support and the electrodes may lie in a plane,as shown in FIGS. 1 and 2.

The solid support may comprise an insulator or a semi-conductor prior tobeing contacted by the analyte. In an embodiment, at least a portion ofthe solid support may be amorphous. For example, from about 5 to about30% by volume of the solid support may be amorphous, or from about 10 toabout 25% by volume may be amorphous. In an embodiment, at least aportion of the solid support may be crystalline. The solid support maycontain one or more amorphous layers in contact with one or morecrystalline layers. Prior to being contacted by the analyte, the solidsupport may be characterized by the absence of a dopant. The solidsupport may be characterized by the absence of any salt moiety. Thesolid support may be porous, with the volume of voids in the poroussolid divided by the total volume of the porous solid being in the rangeup to about 0.7, or from about 0.1 to about 0.7, or from about 0.3 toabout 0.65.

The solid support may comprise a polymer that is a non-conductivepolymer prior to being contacted by the analyte. The solid support maycomprise poly (ethylene terephthalate), poly (ethylene oxide),polyvinylidenefluoride, polyethylene, polypropylene,polyethylene-napthlate, polyphenylenesulfide, polycarbonate,polytetrafluoroethylene, polypropylene oxide, acrylic resin,polystyrene, poly(styrene-acrylonitrile),poly(acrylnitrile-butadiene-styrene), polyvinyl chloride, chlorinatedpolyether, poly(chlorotrifluoro ethylene), or a mixture of two or morethereof. The solid support may comprise glass and/or ceramic. The solidsupport may comprise carbon and/or graphite.

The solid support may comprise any of the above-indicated polymers andone or more fillers. The fillers may be electrically conductive ornon-conductive. The fillers may be inorganic, organic, or a mixturethereof.

The inorganic fillers may comprise one or more silicates, oxides,carbonates, sulfates, hydroxides, carbons, metals, glass, mixtures oftwo or more, and the like. Examples of the inorganic fillers that may beused include clay, talc, mica, asbestos, feldspar, bentonite clay,wollastonite, fuller's earth, pumice, pyrophillite, rottenstone, slateflour, vermiculite, calcium silicate (precipitated), magnesium silicate(precipitated), aluminum oxide, hydrated alumina, antimony trioxide,magnesium oxide, titanium dioxide, zinc oxide, silica, quartz,diatomaceous earth, tripoli, pyrogenic, hydrogel, aeorgel, calciumcarbonate (precipitated), ground limestone, ground marble, bariumcarbonate (precipitated), magnesium carbonate (precipitated), bariumsulfate, barytes, blanc fixe, calcium sulfate, calcium hydroxide,magnesium hydroxide, carbon black, furnace black, lampblack, acetylene,graphite, carbon fibers, metal powders (e.g., copper, aluminum, bronze,lead, zinc, steel), metal fibers, metal whiskers, metal wire, bariumferrite, magnetite, molybdenum disulfide, glass fibers, glass flakes,ground glass, mixtures of two or more thereof, and the like.

The organic fillers that may be used may include ground bark, processedlignin, keratin, soybean meal, nylon fibers, acrylic fibers,fluorocarbon polymer fibers, polyester fibers, wood flour, shell flours,alpha cellulose fibers, cotton flock fibers, sisal fibers, jute fibers,rayon fibers, mixtures of two or more thereof, and the like.

When the fillers are electrically conductive, the amount of filler inthe solid support may be up to about 20% by volume, or in the range fromabout 0.01 to about 20 percent by volume, or from about 0.02 to about 18percent by volume.

In an embodiment, the solid support may comprise one or more of theabove-indicated polymers and one or more of the above-indicatedelectrically conductive fillers, and the volume concentration of the oneor more electrically conductive fillers in the solid support is lessthan the percolation threshold for electrical conductivity of the solidsupport. Thus, for example, if the solid support were to comprisepoly(ethylene terephthalate) (PET) and one or more electricallyconductive fillers, the conductivity of the solid support would bedependent on the conductivity of the PET when the volume concentrationof the electrically conductive fillers in the solid support is below thepercolation threshold for the solid support. On the other hand, if thevolume concentration of the electrically conductive fillers in the solidsupport exceeded the percolation threshold for the solid support, theconductivity of the solid support would be dependent on the conductivityof the electrically conductive fillers.

In an embodiment, the solid support may comprise a crystalline polymerrepresented by the formula

(PEO)₆-LiSbF₆

wherein PEO refers to poly(ethylene oxide). This polymer may beelectrically conductive. While not wishing to be bound by theory, it isbelieved that the polymer chains fold to form cylindrical tunnels insideof which the lithium ions are coordinated by the ether oxygens, whilethe anions are located outside of the tunnels with no coordinatinginteractions with the cations. Enhanced ionic conductivity of thecrystalline material may thus result from the relatively free movementof cations through the tunnels.

The diffusion of the anayte through the solid support to the electrodeinterface may be a rate-limiting step. For a fast response time, thethickness of the solid support may be maintained as thin as possible toprovide for fast diffusion (cm²/s) and yet maintain the desiredmechanical strength properties needed to handle the sensor in bothmanufacturing and in use. The thickness of the solid support may be inthe range from about 0.05 to about 0.6 mm, or from about 0.07 to about0.5 mm, or from about 0.1 mm to about 0.3 mm, or about 0.25 mm.

The inventive amperometric gas sensor may be considered to be anelectrochemical cell in which faraday currents are flowing, causing (orcaused by) chemical reactions at the electrodes, which are separated bythe solid support. The working electrode is where the half reaction ofinterest with the analyte may take place. The reference electrode isessentially nonpolarizable. The electrode reactions are controlled bythe voltage between the metal of the electrodes and the solid support.Control of the potential of the working electrode with respect to thereference electrode may be equivalent to observing or controlling theenergy of the electrons within the working electrode. A more negativepotential at the working electrode (with respect to the referenceelectrode) raises the energy level; a more positive potential (withrespect to the reference electrode) lowers the energy level. If theelectrons reach a high enough energy level, they will transfer from theworking electrode to vacant electronic states in the solid support,creating a flow of electrons from the electrode to the solid support (areduction current). Similarly, if the energy of the electrons is loweredenough, electrons in the solid support will transfer to the workingelectrode (an oxidation current). Oxidation cannot take place withoutreduction, and vice-versa. A feature of the inventive amperometric gassensor is that the simultaneously occurring oxidation-reductionreactions are spatially separated. The electrode reaction may occur inseveral series and parallel reaction steps. There may be three steps ina series: (1) the gaseous analyte is transported to the electrodesurface from the bulk of the solid support (usually predominantly bydiffusion, but it may also occur by electromigration), (2) a chargetransfer reaction occurs, and (3) the product is transported from theelectrode surface to the bulk of the solid support. A potentiostatcircuit may be used for potential control across the electrodes as wellas for measuring the resulting current through the sensor. The magnitudeof the current generated by the electrochemical reaction at the workingelectrode is proportional to the analyte concentration in the gas beingsampled.

A problem with many amperometric gas sensors that are currentlyavailable is that they do not support operation in the vacuum state, asa water reservoir of some sort is typically needed to maintain ionicconductivity in, for example, the Nafion or gel-type electrolytes thatare currently in use. For amperometric gas concentration measurements ina vacuum environment, a solid electrolyte is necessary for leak freeuse, convenient packaging, and sustainability, as any water wouldevaporate quickly. For example, in the vacuum conditions seen in medicalsterilization systems with VHP at 50° C., the vapor pressure of water isabout 11 Torr. The conductivity of Nafion is dependent on humidity,being up to about 1000 times stronger at 100% relative humidity than at20% relative humidity (at room temperature); thus in the vacuum state,performance would vary. Gel electrolytes, too, will vary in conductivitywith humidity at a given temperature, often by more than an order ofmagnitude from saturation to dry air. The inventive amperometric sensorprovides a solution to this problem by being suitable for use inprocesses conducted in a vacuum. Going beyond that, the inventiveamperometric sensor may also be used in processes conducted atatmospheric pressure, and in processes involving pressures aboveatmospheric pressure.

The inventive amperometric gas sensor may be used to determine theconcentration of one or more analytes in the form of oxidizing gases,reducing gases, and the like, that are present in an enclosed space. Theanalyte may comprise vaporous hydrogen peroxide, ethylene oxide, ozone,or a mixture of two or more thereof. The analyte may comprise ahydrogen-containing gas. The analyte may comprise atomic hydrogen,hydrogen sulfide, hydrogen sulfite, ammonia, carbon monoxide, oxalicacid, formic acid, ascorbic acid, phosphorous acid, or a mixture of twoor more thereof. The pressure within the enclosed space may beatmospheric or above atmospheric, for example, an absolute pressure inthe range from about 1 to about 2 atmsopheres. The pressure within theenclosed space may be below atmospheric pressure. The pressure, in termsof absolute pressure, may be in the range from about 0.1 to about 750Torr, or from about 0.1 to about 700 Torr, or from about 0.1 to about600 Torr, or from about 0.1 to about 500 Torr, or from about 0.1 toabout 400 Torr, or from about 0.1 to about 300 Torr, or from about 0.1to about 200 Torr, or from about 0.1 to about 100 Torr, or from about 1to about 75 Torr, or from about 1 to about 50 Torr, or from about 1 toabout 25 Torr, or from about 3 to about 25 Torr, or from about 5 toabout 25 Torr, or from about 5 Torr to about 20 Torr.

The temperature within the enclosed space may be in the range from about15 to about 90° C., or from about 30 to about 70° C., or from about 45°C. to about 65° C., or from about 50° C. to about 60° C.

The inventive amperometric gas sensor may be particularly suited for usein sterilization processes employing a sterilant gas wherein thesterilization is conducted in a vacuum. These sterilization processesmay be particularly suited for sterilizing articles of complex andirregular shapes, for example, articles with narrow apertures, holes,tubes, open ended lumens, internal cavities, deadlegs, flat surfaces,and the like. Numerous medical devices (e.g., endoscopes), dentalinstruments, and the like, are characterized by such complex andirregular shapes.

It is generally desired to sterilize medical devices, dentalinstruments, and the like, before use. In medical and dental facilities,where medical devices and dental instruments need to be used severaltimes per day on different patients, it is important not only tosterilize the instruments between patients to preventcross-contamination, but to do so quickly and economically withoutdamaging the instruments.

Several different methods have been developed for delivering a sterilantgas to the vacuum chamber of a sterilizer for sterilizing medicaldevices, dental instruments, and the like. The sterilant gas maycomprise any sterilant gas that is useful for these sterilizations.These may include vaporized hydrogen peroxide (VHP), ethylene oxide,ozone, or a mixture of two or more thereof. The sterilant gas may bemixed with an alkaline gas such as ammonia. The sterilant gas maycomprise a mixture of vaporized hydrogen peroxide and ammonia. Thesterilant gas may be mixed with a carrier gas. The carrier gas maycomprise air, nitrogen, and the like.

The sterilization process may comprise any sterilization process orsterilization cycle, where the process allows sterilant vapor to becarried into, through, and out of the sterilization vacuum chamber froma vaporizer, with or without carrier gas, during a portion of the cycle.The sterilization process may involve varying vacuum chamber pressuresduring a sterilization cycle or from cycle-to-cycle. For example, thesterilization process may employ a combination deep vacuum/flow-throughcycle, in which the chamber pressure increases as the cycle progressesfrom deep vacuum to flow-through conditions, through a transition phase.The rate of pressure increase, and the actual pressure levels obtainedmay vary during such a cycle or from such cycle-to-cycle, depending, forinstance, on the nature of the instrument load (i.e., the degree of flowrestriction presented by the load).

VHP may be used as the sterilant gas in the sterilization process. TheVHP may be generated from an aqueous solution of hydrogen peroxide. Theaqueous solution may comprise from about 30% to about 40% by weighthydrogen peroxide, and from about 60% to about 70% by weight water. Byadding an alkaline gas that is soluble in the hydrogen peroxide(ammonia, for example), the pH of the sterilant may be controlled. Thepresence of hydrogen peroxide in the sterilant may serve to lower the pH(e.g., 35% aqueous hydrogen peroxide solution has a pH of about 3 toabout 4) and the ammonia may be added to raise the pH to a value ofabout 8 to about 9. The volumetric ratio of VHP to ammonia gas may be inthe range from about 1:1 to about 1:0.0001.

VHP, when used in combination with ammonia gas, may be referred to asmodified VHP or mVHP. VHP and/or mVHP may be effective against microbialand chemical decontaminants because they may provide a broad spectrum ofactivity against a wide variety of pathogenic microorganisms andchemical pathogenic agents, such as hard to destroy spores of Bacillusstearothermophilus, Bacillus anthracis, smallpox virus, and the like.They may be also effective at or close to room temperature (e.g., about15 to about 30° C.), making them suitable for use with little or noheating. VHP and/or mVHP may have good material compatibility.

The pressure within the vacuum chamber during these sterilizations maybe below atmospheric pressure. The pressure, in terms of absolutepressure, may be in the range from about 0.1 to about 750 Torr, or fromabout 0.1 to about 700 Torr, or from about 0.1 to about 600 Torr, orfrom about 0.1 to about 500 Torr, or from about 0.1 to about 400 Torr,or from about 0.1 to about 300 Torr, or from about 0.1 to about 200Torr, or from about 0.1 to about 100 Torr, or from about 1 to about 75Torr, or from about 1 to about 50 Torr, or from about 1 to about 25Torr, or from about 3 to about 25 Torr, or from about 5 to about 25Torr, or from about 5 Torr to about 20 Torr.

The temperature within the vacuum chamber during these sterilizationsmay be in the range from about 15 to about 90° C., or from about 30 toabout 70° C., or from about 45° C. to about 65° C., or from about 50° C.to about 60° C.

Sterilization processes that may be used are described in U.S. Pat. Nos.5,286,448; 5,389,336; 5,445,792; and 5,527,508.

The level of sterilant gas concentration in the vacuum chamber of thesterilizer may determine the level of sterility attained. Thus, knowingthe level of sterilant gas during a sterilization cycle may allow thestate of sterility to be known faster than with post-cycle indicators,such as biological indicators.

In use for hydrogen peroxide detection, the amperometric sensor may beexposed to the sterilant gas in the vacuum chamber of the sterilizer.The gas diffuses through the solid support to the working electrode. Thevoltage at the working electrode with respect to the reference electrodeis set by the potentiostat. For VHP, the voltage is ≧0.68 V. Hydrogenperoxide is oxidized at the working electrode. A corresponding reductionreaction occurs at the reference electrode. Current in the workingelectrode is measured by the potentiostat and provides a quantitativemeasure of the hydrogen peroxide concentration.

The inventive amperometric gas sensor may be used in sterilizationprocesses wherein vaporized hydrogen peroxide alone is introduced andvacuum chamber pressure fluctuates between atmospheric and vacuumlevels. Additionally, in various embodiments the amperometric gas sensormay be exposed to different target gases, and the electrical conditionsmay be selected so that reaction only occurs in the presence of thedesired target gas, and not other gases that may be present in theanalyte gas sample.

It may be desired to effectively and automatically control the amount ofsterilant vapor delivered to a sterilization chamber, during the varyingchamber pressure conditions which may be experienced during asterilization cycle, or from cycle to cycle, to maximize sterilant vaporexposure. It may be desirable, however, that the level of sterilantvapor not exceed its saturation limit under the sterilizationconditions. Otherwise, sterilant will condense, decreasing the amount ofsterilant vapor available for sterilization. Also, condensed sterilant,such as hydrogen peroxide, may degrade or harm the contents of thesterilization chamber. Synthetic materials, such as are employed inflexible endoscopes, for example, may be damaged by condensed hydrogenperoxide.

The inventive amperometric gas sensor may be used to optimize theefficacy of sterilization achieved over a given period of time and/or toshorten the time required to sterilize a variety of instruments, oritems which may be present or loaded into the sterilization chamber orother sealed enclosure. The sterilization process may be particularlysuited for sterilizing endoscopes or other instruments having long,narrow lumens, which may provide varying degrees of flow resistance. Thesterilization process may also be used to sterilize the interior of thesterilization chamber, with or without a load.

The inventive amperometric gas sensor may be used to maximize theconcentration of sterilant gas in the vacuum chamber without exceedingthe saturation limit for the sterilant gas in the sterilization chamber,in response to chamber pressure. By maintaining the concentration ofsterilant gas at a high percentage of its saturation limit, in responseto chamber pressure, cycle time can be reduced and/or greater assuranceof sterilization realized.

By using the inventive amperometric gas sensor, there is no need to waituntil a pre-determined pressure level is reached, before injectingsterilant at a constant rate into the system, to ensure that apre-determined percentage of saturation limit is maintained. Instead,the sterilant can be immediately injected, at a rate adjusted to providethe predetermined saturation limit percentage, in the sterilizationchamber immediately after introduction of the sterilant gas, at thechamber pressure measured prior to injection. Thus, the throughput ofthe sterilization process may be increased, and/or greater assurance ofeffective sterilization over a given period may be provided.

The rate of sterilant gas injected into the flow of a carrier gas may beautomatically adjusted in response to the vacuum chamber pressure, tomaintain a pre-determined maximum percentage of the saturation limit forthe sterilant, during at least a portion of the sterilization cycle inwhich sterilant gas flows through the sterilization chamber with acarrier gas.

The contaminants that may be treated with the sterilization process maycomprise one or more chemical contaminants and/or biologicalcontaminants. Different levels of sterilization may be accomplishedwithin the vacuum chamber. As used herein, the term “sterilization” isintended to encompass both microbial decontamination as well as chemicaldecontamination—the destruction of chemical agents, or their conversionto harmless or odorless compounds. For some applications, thesterilization process that is conducted may be less rigorous than acomplete sterilization, for example, the sterilization may comprisedisinfection, sanitization, decontamination, cleaning, and the like.Sterilization may encompass the neutralizing of unpleasant odors, suchas tobacco smoke, perfume, or body odor residues, and odors and dampnessdue to molds. “Microbial sterilization” may be used herein to encompassthe destruction of biological contaminants, specifically, livingmicroorganisms, and also the destruction or inactivation of pathogenicforms of proteinaceous-infectious agents (prions). The term microbialsterilization encompasses sterilization, the highest level of biologicalcontamination control, which connotes the destruction of all livingmicroorganisms. The term sterilization also includes disinfection, thedestruction of harmful microorganisms, and sanitizing, which connotesbeing free from germs. “Chemical sterilization” is intended to encompassthe destruction of pathogenic chemical agents or their conversion toless harmful or odiferous species.

Exemplary biological contaminants which may be destroyed in thesterilization process include bacterial spores, vegetative bacteria,viruses, molds, and fungi. Some of these may be capable of killing orcausing severe injury to mammals, particularly humans. Included amongthese are viruses, such as equine encephalomyelitis and smallpox, thecoronavirus responsible for Severe Acute Respiratory Syndrome (SARS);bacteria, such as those which cause plague (Yersina pestis), anthrax(Bacillus anthracis), and tularemia (Francisella tularensis); and fungi,such as coccidioidomycosis; as well as toxic products expressed by suchmicroorganisms; for example, the botulism toxin expressed by the commonClostridium botulinium bacterium.

Also included are the less harmful microorganisms, such as thoseresponsible for the common cold (rhinoviruses), influenza(orthomyxoviruses), skin abscesses, toxic shock syndrome (Staphylococcusaureus), bacterial pneumonia (Streptococcus pneumoniae), stomach upsets(Escherichia coli, Salmonella), mixtures of two or more thereof, and thelike. Also included are

Clostridium difficile, Bacillus Stearothermophilus, Clostridiumsporogenes, mixtures of two or more thereof, and the like.

Exemplary pathogenic chemical agents may include substances which areoften referred to as chemical warfare agents, such as poison gases andliquids, particularly those which are volatile, such as nerve gases,blistering agents (also known as vesicants), and other extremely harmfulor toxic chemicals. As used herein, the term “chemical pathogenic agent”is intended to include only those agents which are effective inrelatively small dosages to substantially disable or kill mammals andwhich can be degraded or otherwise rendered harmless by a process whichincludes oxidation.

Exemplary chemical pathogenic agents may include choking agents, such asphosgene; blood agents, which act on the enzyme cytochrome oxidase, suchas cyanogen chloride and hydrogen cyanide; incapacitating agents, suchas 3-quinuclidinyl benzilate (“BZ”), which blocks the action ofacetylcholine; vesicants, such as di(2-chloroethyl) sulfide (mustard gasor “HD”) and dichloro(2-chlorovinyl)arsine (Lewisite); nerve agents,such as ethyl-N, N dimethyl phosphoramino cyanidate (Tabun or agent GA),o-ethyl-S-(2-diisopropyl aminoethyl) methyl phosphono-thiolate (agentVX), isopropyl methyl phosphonofluoridate (Sarin or Agent GB),methylphosphonofluoridic acid 1,2,2-trimethylpropyl ester (Soman orAgent GD).

EXAMPLE

A series of tests are conducted to determine the relationship between(1) electric current readings obtained using amperometric sensorsaccording to the present invention, and (2) hydrogen peroxideconcentration readings obtained using a Guided Wave IRspectrophotometer. The tests are conducted in a sterilizer under vacuumconditions.

The amperometric sensors that are used are illustrated in FIG. 1. Theseare made using palladium sputtered PET films obtained from ConductiveTechnologies, Inc. of York, Pa. Referring to FIG. 1, the solid support22 is a PET film with a thickness of 254 microns, a length of 4 mm and awidth of 3 mm. The electrodes 24A and 24B are palladium electrodes whichare sputtered onto the PET film. Each of the electrodes has a thicknessof 100 nanometers and a width of 1.5 mm. Each electrode spans the 4 mmlength of the PET film. The space 26 between the electrodes 24A and 24Bis 0.88 mm.

The sterilizer is a VHP MD Series Low Temperature Sterilizer supplied bySTERIS. This sterilizer is intended for use in sterilizing medicaldevices, dental instruments, and the like, using VHP as the sterilantunder vacuum conditions. The sterilizer has a vacuum chamber wherein thearticles to be sterilized are placed. The amperometric sensor ispositioned in the vacuum chamber. The sterilization process, whichincludes injecting pulses of VHP into the vacuum chamber, is automated,and includes rapid sterilization cycle times. A microcomputer controlsystem provides for cycle setup, selection, and monitoring.

A Keith ley 6430 Sub Femptoampere SourceMeter with PreAmp is used as apotentiostat. It is connected to the amperometric sensor positioned inthe vacuum chamber via coaxial cable to a vacuum feed through (AccuGlasspart #111326) at a side chamber wall port of the sterilizer. Inside thevacuum chamber, the amperometric sensor is anchored in a Molex 1 mmpitch FFCIFPC Connector Part #0520430619 (using pins 3 and 4). The pinsare soldered to 55 cm of stranded 22 AWG wires that end in AccuGlasspush-on connectors (part #110103) which fit on the vacuum feedthrough.The vacuum feedthrough assembly is kept intact (coaxial connectors andpush-on connectors) when the amperometric sensor is changed. The GuidedWave IR spectrophotometer is positioned on the chamber wall of thesterilizer.

Three or four consecutive VHP cycles are run at the injection amounts of2.1 g, 1.7 g, 1 g of hydrogen peroxide (in the case of the threecycles), and 2.1 g, 1.7 g, 1.3 g, 1 g of hydrogen peroxide (in the caseof the four cycles). Using the fourth pulse of each cycle, equations forhydrogen peroxide concentration (as read by the Guided Wave IRspectrophotometer) as a function of Δ current, as well as for thepressure Δ as a function of the Δ current, are determined for the periodunder vacuum. Also, the Δ in current is plotted as a function of thegrams of injected hydrogen peroxide.

Current flowing through the working electrode is read every 3 secondsthroughout the cycle. The Guided Wave IR spectrophotometer displayshydrogen peroxide readings every two to three seconds. The displayedreading is the average of the last ten seconds of readings. The timeperiod for the data used in the statistical summary is from the momentVHP injection begins to the moment peak current is reached,approximately 200 seconds later. This time period begins at the lastreading before the current and Guided Wave IR spectrophotometer readingsspike upward. The current and Guided Wave IR Spectrophotometer readingsstart to respond within 10 seconds of each other. Pressure drops fromatmospheric level to less than 10 Torr within 160 seconds of startingthe cycle. Vacuum conditions provide a stable relative baseline in eachcycle by “cleaning out” the electrolyte each time. As pressuredecreases, diffusion increases.

Referring to FIG. 4, the performance of the amperometric sensor is shownon the primary y-axis in the vaporized hydrogen peroxide sterilizationsystem, with comparison on the secondary y-axis to the Guided Wave IRspectrophotometer during a sterilization cycle. The Δ of current in theconstant pressure period of each pulse is also tabulated, calculated asthe difference between peak current in the pulse minus the baselinecurrent in the pulse prior to hydrogen peroxide injection. Based on thetabulated maintained average signal strength, any deterioration in theconductivity properties of the electrolyte with exposure is not evident.

Referring to FIG. 5, the performance of the amperometric sensor is shownon the primary y-axis in the vaporized hydrogen peroxide sterilizationsystem, with comparison on the secondary y-axis to the pressure recordedduring the cycle. As the diffusion coefficient of the vaporized hydrogenperoxide passing through the PET to the solid support/electrodeinterface will be a constant value when the pressure is relativelyconstant in the vacuum chamber, and the diffusion coefficient will be adependent variable in the current-time response, the performance of theinventive amperometric sensor is considered valid when pressure is notchanging. Thus the fluctuation spike in current that occurs with thepressure change at the end of each pulse from vacuum up to atmosphericpressure as shown, is in accordance with theory.

The response of the amperometric sensor and the Guided Wave IRspectrophotometer readings are shown in FIGS. 6 through 9. FIGS. 10through 12 show that a similar exponential response with R² valuesexceeding 0.996 are obtained for the sensors that undergo at least 3consecutive cycles of varying injection amounts. Equations for theGuided Wave IR spectrophotometer reading y as a function of the changein current x are as follows:

y=1.6607In(x)−2.6361R ²=0.9958   (Equation I)

y=7417In(x)−2.2508R ²=0.9996   (Equation 2)

y=1.7735In(x)−3.0668R ²=0.9983   (Equation 3)

Using the fourth pulse of the cycles, the equations for the VHPconcentration (as read by the Guided Wave IR spectrophotometer) as afunction of Δ current, as well as the pressure Δ as a function of the Δcurrent, are calculated with Excel and Matlab for the period undervacuum. Excel is a software program available from Microsoft. Matlab isa software program available from MathWorks of Natick, Mass. The resultsare shown in FIGS. 10-12. The graphs in FIGS. 10-12 correspond to theraw data of FIGS. 6-8, respectively. Two different equations forpressure, denoted “begin” and “end”: show a slight difference when themore conservative approach is taken of reading the Δ in pressure at thebeginning of the vacuum period, in comparison to reading it at the endof the vacuum period where it is within 1 Torr higher.

The Δ in current in pulse 4 as a function of the grams of hydrogenperoxide injected into the sterilizer is shown in FIGS. 13-15. Thevalues plotted in FIGS. 13-15 are taken from the raw data of the threeconsecutive cycles in FIGS. 6-8, respectively. The exponential functionis fitted with Excel.

FIG. 16 is a linear-log graph representation which shows an example ofthe straight line response obtained for FIGS. 10 through 15. Actualcurrent readings in real-time are measured every 3 seconds. The samplingtime used in the Guided Wave's IR spectrophotometer calculation of theconcentration is the last 10 seconds of data. The peak values of theGuided Wave IR spectrophotometer readings for a pulse are used in allmathematical equation calculations. The Δ in current for the pulse isused for all the mathematical equation calculations rather than a directcurrent reading to eliminate background current effects.

The following Table 1 summarizes the results and shows a statisticalsummary of how well Equation 2 (y=1.7417Ln(x)−2.2508), predicts theactual peak Guided Wave IR spectrophotometer reading for the totalchange in current, I in pA. Values shown in Table 1 are for the final(4th) pulse of the cycles run.

TABLE 1 % Error = % Difference = (Actual Guided | ((Actual Guided WaveWave—Predicted Concentration)— Concentration)/ (Predicted ((ActualGuided Concentration)/ Wave Actual Guided (Actual Guided Concentration +Wave Grams Predicted Wave Predicted Test ΔI, Concentration, of H₂O₂Concentration, Concentration Concentration)/ Number pA mg/L Injectedmg/L, Eq. 2 )) × 100 2)) × 100) | 1 332.5 7.1 2.1 7.9 10.7 10.2 2 164.35.7 1.7 6.6 16.4 15.2 3 36.9 3.4 1.0 4.0 18.6 17.0 4 202.0 7.0 2.1 7.00.1 0.1 5 103.0 5.8 1.7 5.8 0.4 0.4 6 26.0 3.4 1.0 3.4 0.7 0.7 7 42.04.3 1.3 4.3 1.0 1.0 8 211.8 7.1 2.1 7.1 0.3 0.3 9 107.9 5.9 1.7 5.9 0.00.0 10 299.0 7.1 2.1 7.7 8.1 7.8 11 147.3 5.7 1.7 6.4 13.1 12.3 12 37.73.4 1.0 4.1 19.7 18.0

Table 1 shows the peak Guided Wave IR spectrophotometer concentration(mg/L) value predicted by Equation 2 for the total change in current forthe fourth pulse in each of the samples of the experiment. The actualGuided Wave IR spectrophotometer readings for the pulses are analyzedwith Minitab statistical software (available from Minitab Inc. of StateCollege, Pa.). As shown through the remaining Minitab summaries andplots, all but 2 currents are within one standard deviation of the meancurrent at each injection amount (grams), and all predicted values arewithin a 20% error margin of the value actually obtained by the GuidedWave IR spectrophotometer for the pulse.

Descriptive Statistics: I, pA

Grams SE St injected N N* Mean Mean Dev Minimum Q1 Median Q3 Max 1.0 3 033.53 3.77 6.54 26.00 26.00 36.90 37.70 37.70 1.3 1 0 42.00 * * 42.00 *42.00 * 42.00 1.7 4 0 130.6 15.0 30.0 103.0 104.2 127.6 160.1 164.3 2.14 0 261.3 32.2 64.4 202.0 204.4 255.4 324.1 332.5

Descriptive Statistics: Actual Guided Wave IR Spectrophotometer

Grams Injected N N* Mean SE Mean St Dev Coef Var Min Q1 Med Q3 Max 1.0 30 3.400 0.000 0.000 0.00 3.40 3.40 3.40 3.400 3.40 1.3 1 0 4.300 — — —4.30 — 4.30 — 4.30 1.7 4 0 5.775 0.047 0.095 1.66 5.70 5.70 5.75 5.8755.90 2.1 4 0 7.075 0.025 0.050 0.71 7.00 7.025 7.10 7.100 7.10

Descriptive Statistics: Predicted Concentration

Grams Injected N N* Mean SE Mean St Dev Coef Var Min Q1 Med Q3 Max 1.0 30 3.833 0.219 0.379 9.88 3.40 3.40 4.00 4.10 4.10 1.3 1 0 4.30 * * *4.30 * 4.30 * 4.30 1.7 4 0 6.175 0.193 0.386 6.25 5.80 5.825 6.15 6.556.60 2.1 4 0 7.425 0.221 0.443 5.96 7.00 7.025 7.40 7.85 7.90The results indicate that none of the predicted concentrations (mg/L)using Equation 2 overlaps with the results from the inventiveamperometric gas sensor, This means that injection amounts of 1 g, 1.3g, 1.7 g, and 2,1 g of hydrogen peroxide would be uniquely discerniblewith the inventive amperometric gas sensor.

While the disclosed invention has been explained in relation to variousdetailed embodiments, it is to be understood that various modificationsthereof may become apparent to those skilled in the art upon reading thespecification. Therefore, it is to be understood that the inventionspecified herein is intended to include such modifications as may fallwithin the scope of the appended claims.

1. An amperometric gas sensor for measuring a concentration of ananalyte, comprising: a solid configured as an insulator without beingcontacted by the analyte and configured for diffusion of the analytetherethrough, the solid comprising a non-conductive polymer, the solidfurther configured to increase in electrical conductivity when incontact with the analyte; a working electrode positioned on and incontact with the solid; and a reference electrode positioned on and incontact with the solid, the reference electrode spaced apart andinsulated from the working electrode without the solid being contactedby the analyte, the working electrode and the reference electrodeconfigured to measure electrical conductivity of the solid when thesolid is in contact with the analyte.
 2. The sensor of claim 1 whereinat least a portion of the solid is amorphous.
 3. The sensor of claim 1wherein at least a portion of the solid is crystalline.
 4. The sensor ofclaim 1 wherein the solid comprises a porous solid, the volume of voidsin the porous solid divided by the total volume of the porous solidbeing in the range up to about 0.7.
 5. The sensor of claim 1 wherein thesolid comprises poly (ethylene terephthalate), poly (ethylene oxide),polyvinylidenefluoride, polyethylene, polypropylene,polyethylene-napthlate, polyphenylenesulfide, polycarbonate,polytetrafluoroethylene, polypropylene oxide, acrylic resin,polystyrene, poly(styrene-acrylonitrile),poly(acrylnitrile-butadiene-styrene), polyvinyl chloride, chlorinatedpolyether, poly(chlorotrifluoro ethylene), or a mixture of two or morethereof.
 6. The sensor of claim 5 wherein the analyte comprises anoxidizing gas or a reducing gas.
 7. The sensor of claim 5 wherein theanalyte comprises vaporous hydrogen peroxide, ethylene oxide, ozone, ora mixture of two or more thereof.
 8. The sensor of claim 5 wherein thewherein the analyte comprises vaporous hydrogen peroxide.
 9. The sensorof claim 5 wherein the analyte comprises hydrogen sulfide, hydrogensulfite, ammonia, methane, ethane, propane, butane, carbon monoxide,oxalic acid, formic acid, ascorbic acid, phosphorous acid, or a mixtureof two or more thereof.
 10. The sensor of claim 1 wherein the solidcomprises poly (ethylene terephthalate).
 11. The sensor of claim 10wherein the analyte comprises an oxidizing gas or a reducing gas. 12.The sensor of claim 10 wherein the analyte comprises vaporous hydrogenperoxide, ethylene oxide, ozone, or a mixture of two or more thereof.13. The sensor of claim 10 wherein the wherein the analyte comprisesvaporous hydrogen peroxide.
 14. The sensor of claim 10 wherein theanalyte comprises hydrogen sulfide, hydrogen sulfite, ammonia, methane,ethane, propane, butane, carbon monoxide, oxalic acid, formic acid,ascorbic acid, phosphorous acid, or a mixture of two or more thereof.15. The sensor of claim 1 wherein the solid further comprises anelectrically non-conductive filler.
 16. The sensor of claim 1 whereinthe working electrode comprises gold, platinum, iridium, palladium,osmium, silver, rhodium, ruthenium, titanium, or a mixture of two ormore thereof.
 17. The sensor of claim 1 wherein the reference electrodecomprises gold, platinum, iridium, palladium, osmium, silver, rhodium,ruthenium, titanium, or a mixture of two or more thereof.
 18. The sensorof claim 1 wherein the analyte comprises an oxidizing gas or a reducinggas.
 19. The sensor of claim 1 wherein the analyte comprises vaporoushydrogen peroxide, ethylene oxide, ozone, or a mixture of two or morethereof.
 20. The sensor of claim 1 wherein the analyte comprisesvaporous hydrogen peroxide.
 21. The sensor of claim 1 wherein theanalyte comprises hydrogen sulfide, hydrogen sulfite, ammonia, methane,ethane, propane, butane, carbon monoxide, oxalic acid, formic acid,ascorbic acid, phosphorous acid, or a mixture of two or more thereof.22. The sensor of claim 1 wherein the solid comprises poly(ethyleneterephthalate), the working electrode comprises palladium, and theanalyte comprises vaporous hydrogen peroxide.
 23. The sensor of claim 1wherein the solid is in the form of a poly (ethylene terephthalate) filmwith a thickness in the range from about 0.05 to about 0.6 mm.